When a person smiles or frowns, you usually know what it means. The human face expresses a wide range of moods and emotions. The face of a monkey or ape expresses many emotions, too –but it’s easy for a human observer to be fooled.

A British scientist named Jane Goodall studied the facial expressions of wild chimpanzees in Africa. As she watched the animals, she learned to recognize feelings like fear, anger, joy and frustration in a chimp’s mobile face. For instance, if a chimp is angry or intends to attack, it presses its lips together tightly. If it’s in a friendly mood and wants to be groomed by a companion, it purses its lips and pushes them forward in a pout. In the chimpanzee’s world, a pout is both a greeting and an invitation.

When a chimp opens its mouth, pulls back its lips, and shows its teeth, it appears to be smiling or grinning. But its grin isn’t assign of pleasure. Instead, a grinning chimp reveals fear. A chimp grins during and after an attack, or when a stronger chimp threatens it. Grinning is almost always accompanied by loud screaming. History doesn’t record whether Ham the astrochimp was screaming when he landed on earth, but his wide grin showed clearly that he was scared out of his wits.

If a chimp keeps its teeth closed when it grins, it is more nervous than frightened. This kind of grin is accompanied by squeaking sounds and whimpers. A low-ranking chimp approaches a higher-ranking one by displaying a closed grin, which is very much like a nervous smile in a human.

A chimp does have a genuine smile, what Jane Goodall called a “play-face”. When a chimp is happy or having fun, it opens its mouth, juts out its chin, and shows its lower teeth. This play-face is accompanied by grunting sounds, or chimpanzee laughter.